1. Field of the Invention
This invention relates to structural panels made of laminated composite material used for the construction of aircraft fuselages. More specifically, the invention relates to a structural aircraft panel subjected to high-energy impacts, the special structure of which can prevent the impacting projectile intruding beyond a predetermined distance inside the fuselage.
2. Description of the Related Art
It is known that the use of composite materials enables lighter structures to be produced for the same mechanical performance. This is especially advantageous in the case of aircraft structures.
In many cases, low- or medium-thickness composite structures, such as the skin of an aircraft fuselage, nacelle panels or the panels of the landing gear housing, do not permit high incident energy or high-speed projectiles to be contained.
Unlike metallic materials, which can dissipate the energy through plastic deformation, composite materials have a brittle behavior under impact, which means that the material's absorbent potential cannot be used in a resilient way. Therefore it is traditionally recommended to significantly thicken the laminate so as to avoid rupture in the areas where the structure must protect vital aircraft systems.
However, in most cases, this solution leads to other problems:                Introduction of local rigidity gradients: the interruption of layers can cause harmful delamination (separation of layers) under impact and particularities may appear during dimensioning under static loads and fatigue.        Introduction of too great a thickness, causing transverse traction effects through wave reflection, resulting in delamination that can propagate.        More complex manufacturing: handling the extra thicknesses requires changes in the production cycle.        Increased mass        
On the other hand, solutions for protection by shielding (combining several absorbent or resistant materials in a laminate) as a secondary structure, as disclosed in international patent application WO 2006/070014, are disadvantageous in several respects:                The addition of an additional mass, non-stress-bearing with regard to operational stresses                    Durability of the shielding elements in the environmental conditions experienced by the aircraft            Changes to the manufacturing process and the inspection and maintenance procedures.                        
There is therefore a need for a structural aircraft panel incorporating protection against high-energy impacts.
However the only deformation mechanisms able to dissipate energy in composite materials are damage modes.
Patent application U.S. 2007/095982 describes a structural aircraft panel formed from a composite material with fiber reinforcement and able to withstand impacts such as collisions with birds. In this case the skin is made of a composite material specially optimized to withstand shocks and not break during these impacts but instead deform and deflect the trajectory of the impacting body. This solution is effective in cases of impact with a projectile such as a bird, which behaves like a viscous fluid and whose impact energy is distributed over a large area of the panel. The solution is not effective against impacts with debris that generally impacts over a small area.